EzSBC Electronic Products and Discussion

Monthly Archives: June 2015

I recently described a board for hacking on OpenWrt and that also has connectors for adding USB peripherals and custom made PCBs. The AR9331 board is described in the post immediately preceding this one. Here I will describe the connectors in more detail.

The board has the following connectors:

Power Input

Two RJ45 Ethernet Ports

2 Type A USB ports

Mini-USB Port

WiFi Antenna Port

The Expansion Connector

The Power Input is used for powering the board and the peripherals attached to the board including the USB Type A ports. The board has a switch mode regulator to convert the input voltage to 5V at high efficiency. A standard barrel socket is used with the center pin connected to positive and the outer sleave being ground. The ideal input voltage is 12V since the efficiency is very high at 12V input but any voltage from 7V to 17V will work. The 7V lower limit is dictated by the maximum duty cycle of the switching regulator. The switching regulator is rated for 20V continuous but I prefer to not use input voltages higher than 17V to prevent spikes from killing the regulator. The input voltage will spike if the power is unplugged while the board is drawing significant current. There is a 1A PolyFuse is series with the input voltage to protect the PCB from damage if there is a short circuit somewhere on the board or USB plugs.

The RJ45 ports have integrated magnetics and the tabs for the cable connector point downwards. Ordinary Cat5 and Cat5E cables can be used.

The two USB ports can both act as USB Host ports since there is a USB Hub on the board. These USB ports can power external hard drives and other devices that draw significant current from the 5V supply. The 5V in the USB is current limited at about 3A for both ports combined. The power is available even if the device has not been enumerated. Since these boards are aimed at being embedded inside other products the 5V from the USB can power external 5V devices including battery chargers. We have tested several USB hard drives, flash drives and some other peripherals on the USB ports. One of the devices known to work is a USB to Serial adapter based on an FTDI chip. Similar boards normally can’t drive USB to Serial or USB to Parallel converters due to the compatibility issues of the AR9331 chipset with USB 1.1 devices.

The Mini-USB port has a FT231SX USB to serial adapter between it and the AR9331’s integrated UART. This serial port is used by the Linux kernel as an error reporting channel. You can log in using the root password and change setup of the WiFi and firewall tables. Also, if all else fails, you can upload new firmware through this port. Normally the firmware would be uploaded via the Ethernet or WiFi ports. A change in the build settings for OpenWrt can make this serial port available for general use.

The WiFi port is a 50 Ohm 2.4GHz port for transmitting and receiving the WiFi signals. The connector is a standard IPAX u.FL connector and you can connect any 50 Ohm antenna to the module. These connectors are not designed for many mating cycles and a typical spec is 10 cycles. The boards will be supplied with an antenna that is printed on a PCB but any standard 2.4GHz antenna will work.

The Expansion Connector is a 20-pin 0.1″ header connector and is not fitted on the modules. The connector has a a number of signals that connect directly to the AR9331 SOC and they should be treated with care. The AR9331 is a 2.5V device and its pins are NOT 3.3V or 5V tolerant.

1

GPIO15

GPIO18

2

3

GPIO16

GPIO19

4

5

Vin

GPIO20

6

7

Gnd

GPIO21

8

9

RXftdi

GPIO22

10

11

TXftdi

GPIO23

12

13

2.5V

GPIO24

14

15

3.3V

GPIO26

16

17

5V

GPIO27

18

19

Gnd

GPIO28

20

Some of the pins have special functions assigned to them with hardware support for those functions and I will cover them later. The 2.5V pin is provided more as a reference than a power supply pin. You should not draw current from it. The 3.3V pin can suply some current and I will qualify it in more detail in the future. It is limited by the fact that there is no heatsink on the 3.3V LDO except for some copper areas attached to the device. It this point I think 50mA should be available with a possibility of 100mA or even 150mA. The 5V pin is the primary regulated supply and any board that needs more than a few mA should have a regulator from the 5V to generate its own supply voltage. In the event that a module needs a voltage higher than 5V I routed the input voltage directly to the Vin pin but it is unregulated and untouched except for the PolyFuse protection.

GPIO15 is shared with a signal called LED4 on the AR9331 and the description reads “Ethernet switch LED3” and it defaults to a general purpose IO pin.

GPIO16 is shared with LED5 for which there is even less information available but it defaults to an IO pin.

GPIO18 can be configured as I2S_CK for an I2S audio inteface.

GPIO19 can be configured as I2S_WS for an I2S audio inteface.

GPIO20 can be configured as I2S_SD for an I2S audio inteface.

GPIO21 can be configured as I2S_MCK for an I2S audio inteface.

GPIO22 can be configured as I2S_MICIN for an I2S audio inteface.

GPIO23 can be configured as SPDIF_OUT for a digital audio output.

GPIO24 is always a general purpose IO-pin.

GPIO26 is a general purpose IO-pin that can be configured as a generic LED driver.

GPIO27 is a general purpose IO-pin that can be configured as a generic LED driver.

GPIO28 is a general purpose IO-pin that can be used to control and external Low Noise Amplifier for increased WiFi sensitivity. It is always available as an IO pin on this board.

The I2S Interface

I2S/SPDIF out is an audio interface that support up to 48 KHz sampling clock and a serial clock of more than 512 * sampling frequency. It also supports seamless switching of the audio out stream from I2S to SPDIF. I2S MIC is also supported. Can generate serial clock for various sampling frequencies.

Dragon Development Board

It is a low cost development board that lets you embed Linux into your projects. It runs OpenWrt with a Linux kernel, has 2 USB host ports and has full Ethernet and 802.11 b/g/n WiFi capabilities.

Applications:

Internet of Things

Voice over IP

Mesh WiFi

Industrial Control

Internet Radio

3G/4G router

OpenWrt is a reliable Linux distribution for embedded systems with an emphasis on networking. It is open source, actively developed and well maintained. OpenWrt uses the OPKG package system, has lots of powerful utilities and the software is maintained in package form. Users can easily install, add and remove packages to make a very customized system.

Why?

The board was developed as an alternative to hacking a commercial router such as the TP-Link TL-WR703N Mini Portable Wireless Router to use as a development platform. There are many routers available based on the Atheros AR9331 Chipset and running customized versions of OpenWrt or DD-Wrt software. The TL-WR703N is popular because of its low price and small size.

Hacking a router to install OpenWrt is a well tried path to getting a functional board with open source software to allow development of low volume or one-off projects. It has its problems. In most cases the schematics are not available unless someone took the trouble to reverse engineer the schematics from the printed circuit board. The TL-WR703N has been reverse engineered and the schematics are available here . I looked at the schematics and they appear reasonable and are most likely correct or nearly so. So again, why bother making a board based on the same chipset? Part of the answer lies in our own requirements. We wanted a board with more than one Ethernet port and a USB port to allow us to write data on a real hard drive. The Raspberry Pi and the Beaglebone fail the two Ethernet port requirement although they have lots of USB ports available. In some cases we want to turn off the radio and use only the wired ports for security reasons. Also, routing Ethernet packets without hardware assistance is a CPU killer in terms of work load. That still leaves the option of using a commercial router and hacking it.

There are many routers out there and may are well supported by OpenWrt, our preferred operating system. Many have four or five Ethernet ports and some have USB ports as well. Some testing has shown that we need 64M of RAM for our target application and that ruled out some routers. The lower cost routers all seemed to be based on the AR9331 system on a chip and some testing on hacked boards lead to a remarkably annoying discovery. You can have two identical routers with identical software and one will have a reliable USB port, one not. Even worse, some have very low failure rates so they appear OK but will hang when left unattended.

Searching on the Internet for a fix showed that OpenWrt has a patch for the problem. After making sure the patch was applied we tested a couple of boards and they worked well. In fact, they worked so well that we considered the problem solved and started developing the application. After some time a board that worked correctly acted up and would not be reliable with a hard disk attached to the AR9331 USB port. We had previously tested this exact board with a hard drive and copied very large amounts of data from one place on the drive to another. We also read the content of the entire drive, a few hundred gig without any problem whatsoever. I tried another board with a physical hard drive and it worked fine but it absolutely refused to read an old USB Flash drive. Searching on the Internet for a solution I saw a post somewhere that installing a passive USB hub between the chipset USB port and the drive (or keyboard, USB to UART adapter or some other devices) fixes the problem for some people. We tried it and it worked! A flaky USB system became reliable. Two questions remained; Is it really fixed? and What is the cause? There is a large topic on the OpenWrt forum that started in 2012 and is still going about this USB issue. https://forum.openwrt.org/viewtopic.php?id=39956 There were many theories and suggestions including reading undocumented registers and writing the contents back periodically. More searching and emails eventually crystallized into the following facts:

All AR9331 devices have a USB stability issue

It is not power supply related

Disabling the WiFi makes the problem go away

The problem can be solved by adding a USB 2.0 Hub between the AR9331 and the offending devices

USB 1.1 devices never work reliably.

The problem seems to be due to a hardware bug in the AR9331 and terminology problems. The AR9331 has a USB2.0 controller and it works reliably with USB 2.0 devices (only) but is unreliable with USB 1.1 or Full Speed USB devices. The problem can be resolved in hardware by installing a hub that connects to the AR9331 as a high speed USB 2.0 device and translates USB 1.1 transactions to USB 2.0 transactions.

I added a USB 2.0 Hub to the board and for good measure I added a second USB host port as well. We now have reliable USB ports for anything recognized by the software. Since one of our aims is to use physical hard drives attached to the USB ports I added a 5V 3A switch mode regulator to the board and wired the 5V directly to the two USB host ports. The ports don’t respect the 100mA/500mA current limits of standard USB port and will happily supply 2A or more on a port as long as the total current drain is less than 3A.

The board has been undergoing testing now for a couple of weeks and it appears that all the bugs are now in the software. All the parts are in house but the solder paste stencil is still in process. The board will be available for purchase by the end of June 2015 and a single unit will be below $50.